A police radar unit transmits microwaves of frequency 3 ×

A police radar unit transmits microwaves of frequency 3 × 1010 Hz. The speed of these waves in air is 3.0 × 108 m/s. Suppose a car is receding from the stationary police car at a speed of 140 km/h. What is the frequency difference between the transmitted signal and the signal received from the receding car?